Typically, there are five main techniques to achieve an extended depth of field referred to as EDOF.
The first technique employs an optical element, referred to as phase plate, inserted in the optical system in order to make blur uniform in the scene depth direction. Then, based on a previously-measured blur pattern or a simulation-calculated blur pattern, the technique executes image-restoration processing on an image obtained through the blur uniformity. Hence, the technique generates an EDOF image.
This technique is introduced as the wave-front coding, or WFC, as disclosed in Non-patent literature 1.
The second technique employs an aperture of which pattern is modified, so that the distance to the focal plane is accurately measured for each of sub-regions of the image. Then, the technique executes image-restoration processing on each sub-region, using a blur pattern which is based on each of previously-measured distances to a corresponding one of the sub-regions. Hence, the technique generates an EDOF image. This technique is introduced in Non-patent literature 2 as the coded aperture, or CA.
The third technique involves shifting a focus lens or an imaging device during the exposure time in order to convolve images which are uniformly focused in the scene depth direction. Then, based on a previously-measured blur pattern or a simulation-calculated blur pattern, the technique executes image-restoration processing on the image obtained through convolution. Hence, the technique generates an EDOF image. This technique is introduced in Non-patent literature 3 as the Flexible DOF, or F-DOF.
The fourth technique, as disclosed in Non-patent literature 4, involves estimating the depth and detecting the sharpness of the image, taking advantage of the on-axis chromatic aberration, and generating an all-focus image through image processing.
The fifth technique, as disclosed in Non-patent literature 5, involves making uniform blur in the scene depth direction using a multifocal lens, and executing image-restoration processing on the image obtained through the uniformity using a previously-measured blur pattern or a simulation-calculated blur pattern.
The F-DOF employs a lens whose light-collecting area is optimized to convolve images which are uniformly focused in the scene depth direction. Hence, in principle, the F-DOF provides excellent images. One of the largest advantages of the F-DOF is that, with the F-DOF, a high-quality image can be obtained more easily than with another technique used for obtaining an image in a less-ideal light-collecting environment created due to the inserted phase plate, the modified aperture pattern, the changed chromatic aberration, and the purposefully created blur with the multifocal lens.
However, Non-patent literature 6 shows that, as an optical condition to the F-DOF, the same object needs to be convolved on the same position of the image even though the focal point shifts during the exposure. Consequently, the F-DOF inevitably requires an additional image-side telecentric lens.
The oldest application of the above EDOF technique to industrial products is the one to the microscope.
In the case of the microscope, the technique that has long been used is to generate an EDOF image from multiple images, because a user can take time to obtain an image of a still object. The technique, however, requires much time and work as described above. Hence, along with the above technique, ideas based on the F-DOF technique have been disclosed in some references, such as Patent literatures 1 to 4. There are two disclosed techniques to employ the F-DOF for the microscope. The first technique is to move a specimen; namely the object, during the exposure, and the second technique is to move the lens barrel during the exposure.
Recently, the EDOF technique has also been applied to a camera for cellular phones. The use of the EDOF technique for the camera contributes to making the camera smaller. In other words, the EDOF effect eliminates the need of an autofocus system for obtaining an all-focus image; that is all the objects in the image are focused.
In view of the applications of the above technique, the F-DOF itself is not adopted. This is because the F-DOF requires a mechanism to shift the focus lens or the imaging device, which makes harder to manufacture the camera smaller at a lower cost. Instead, adopted is the WFC or the technique utilizing the on-axis chromatic aberration.
Another promising application of the EDOF technique is to the one to digital still cameras.
In recent years, users have been looking for more user-friendly and further foolproof digital still cameras. The EDOF technique can meet such a request, and is promising since the technique makes it possible to obtain an all-focus image, freeing a user from obtaining an out-of-focus image.
In addition, the EDOF technique is effective when the user captures an object in a short distance, such as macro photography. For example, the EDOF technique successfully compensates a drawback that the range of focus is extremely short in the macro photography of flowers and insects. The EDOF technique can also meet the needs for a more extended depth of field.
In the above applications, the following features are required: As a basic requirement, the image should be restored in high quality; the EDOF should be large enough to obtain an all-focus image; the range of the EDOF should be able to be changed at the user's option; and, furthermore, the capturing options should be able to be easily switched between the EDOF capturing and regular image capturing so that the user can also chose the regular image capturing. The technique to satisfy all the requests among the above techniques is the F-DOF, which is very promising.
Described next are the details of the F-DOF technique.
As shown in Non-patent literature 3, the F-DOF technique involves moving the imaging device or the lens barrel in order to convolve images which are uniformly focused in the scene depth direction.
Suppose the case where the imaging device is moved to obtain the EDOF image. Here, with the lens barrel fixed to the holding stand, the imaging device is accurately moved in the direction of the optical axis of the lens barrel using the power generated by a microactuator. Such an operation changes the distance between the imaging device and the lens barrel, which makes it possible to convolve images which are uniformly focused in the scene depth direction.